(i) Field of the Invention
The present invention relates to a receiver which can receive an FM multiplex broadcast of DARC system, especially to a synchronous circuit which includes a forward protection circuit or a rearward protection circuit.
(ii) Description of the Related Art
For FM multiplex broadcasting, RDS (radio data system) is most common in Europe, while, in Japan, DARC (data radio channel) systems are most common.
RDS data include (1) program identification data (PI data), (2) broadcasting station name data (PS data), (3) a list of frequencies of other broadcasting stations which are broadcasting the same program (AF list), (4) program content identification data (PTY), (5) identification data of music or conversation (M/S data), (6) identification data of traffic information station (TP), (7) identification data of broadcasting of traffic information (TA) and the like.
Currently, many vehicle-mounted RDS receivers have a search function which uses this data to find another station broadcasting the same program when a receiving condition of the present station deteriorates as the vehicle moves. For example, in an AF search function, the list of the frequencies at which the same program is broadcast (AF list) is demodulated, a receiving frequency is successively switched to the stations of the frequencies on the list, the receiving electric field strengths of the stations are checked, and an AF station with a maximum receiving electric field strength is detected. Subsequently, the PI data of the detected AF station is demodulated, and it is judged whether or not the demodulated PI data and the PI data of the present station actually coincide. If they do coincide, the program is received from the AF station with the maximum receiving electric field strength. If they do not coincide, the PI data of an AF station with the next large receiving electric field strength is demodulated and it is judged whether or not the demodulated PI data and the PI data of the present station coincide. Thereafter, this operation is successively performed. The determination of the PI data is necessary because in the AF list some stations actually broadcast no RDS or a large number of stations have the same frequency. The PI data often varies.
As shown in FIGS. 1 and 2, RDS data is repeatedly transmitted in one group of 104 bits. One group consists of four blocks, each of 26 bits, and each block consists of a data portion of 16 bits and 10 check bits. A bit rate is 1.1875 KHz. One block is transmitted in about 22 msec. Therefore, one group is repeatedly transmitted in about 88 msec. The RDS data transmission system includes A and B versions. In version A, the PI data is allocated to a first block in the four blocks. Therefore, a repetition cycle of the PI data is about 88 msec. In version B, the PI data is allocated to first and third blocks in the four blocks. Therefore, the repetition cycle of the PI data is about 44 msec. Therefore, when AF search including the detection of the PI data is performed, in order to demodulate the PI data of the AF station, in addition to the repetition cycle, about 60 msec of RDS data block synchronization time is also necessary.
On the other hand, as shown in FIG. 3, one block of DARC data contains 288 bits. Further, 272 blocks constitute one frame, which is repeatedly transmitted. In 272 blocks, 82 blocks are used for correcting errors (vertical parity), while the remaining 190 blocks are used as information blocks. Each information block of 288 bits consists of 82 bits for correcting errors (horizontal parity), 14 bits of CRC for detecting errors, 176 bits of information and 16 bits of BIC (block identification code) for use in detecting synchronization of blocks and frames. Each error correcting block of 288 bits consists of 272 bits for correcting errors and 16 bits for BIC. The bit rate is 16 KHz. One block is transmitted in 18 msec. Therefore, one frame is repeatedly transmitted in about five seconds.
In Europe, some broadcasting stations are planning to deliver FM multiplex broadcasts in both the RDS and DARC systems. Therefore, if a RDS receiver having the AF search function which includes the detection of the PI data is additionally provided with an FM multiplex demodulating function of the DARC system while a front end is common to both systems, then for about 150 msec during the AF search in the RDS, DARC data as well as voice from the present station are discontinuous.
A block synchronous circuit and a frame synchronous circuit are required to demodulate the DARC system data. Each of the synchronous circuits has a forward protecting function which has a threshold value for holding a synchronous condition, even when the data cannot be exactly received because of noise. However, when the threshold value is exceeded and the data cannot be exactly received, the circuit becomes unsynchronized. Once a deviation from the synchronization occurs, a considerable amount of time is necessary to reestablish synchronization (rearward protection).
Therefore, if DARC data is discontinued for 150 msec, the BIC for synchronization cannot be detected in the DARC data of continuous eight or more blocks. Even if the forward protecting function is provided, there is a possibility that the DARC receiver is out of synchronization with the present station. Once synchronization is deviated, the received data cannot be processed or errors cannot be corrected until the synchronization is again established. During the period, the received data is ineffective.
Also, the block synchronous circuit and the frame synchronous circuit are provided with rearward protection circuits in which, even if there are about one or two bits of incorrect BIC due to noise, a non-synchronous condition can be changed to a synchronous condition by detecting BIC several times at a predetermined timing. However, once the rearward protection circuit brings back the synchronous condition with the incorrect data, an incorrect synchronous condition is continued by means of the function of the forward protection circuit.
Therefore, when AF search is performed as described above, another station is received while the receiver is not synchronous with DARC data in the present station. Then, the synchronous circuit is possibly brought back into the synchronous condition with the DARC data of another station and the noises. In this case, even if the frequency is adjusted back to that of the present station, the synchronous circuit is operated at the data timing of another station. An incorrect lock condition is continued until the forward protection circuit determines that a deviation from synchronization occurs. In the incorrect lock condition, the block synchronization or the frame synchronization cannot be correctly performed. Therefore, correct data cannot be obtained. During the period, the received data is useless.
Further, the FM multiplex data of the DARC system also includes the AF (alternative frequency) list in the same manner as the RDS. Even vehicle mounted DARC systems will be provided with a search function similar to the AF search function of the RDS receiver. Therefore, as well as in car mounted receivers DARC only systems, when AF or another search function is performed, the same problem will arise as described above.
An object of the present invention is to provide an FM multiplex broadcast receiver which prevents deviation from DARC synchronization, even when DARC data is discontinued during AF search or the like, and which can effectively receive the DARC data.
Another object of the invention is to provide an FM multiplex broadcast receiver which prevents a DARC synchronous circuit from being incorrectly locked, even when a receiving frequency is once changed to a frequency other than a frequency of a DARC data receiving station during AF search or the like, and which can effectively receive the DARC data.
According to the present invention, when an AF search including detection of PI data in RDS or another search is performed and the DARC data is long discontinued, no deviation from DARC synchronization is caused. Therefore, the data received immediately after the search is completed can be processed and errors can be corrected. No received data is made ineffective. The invention is effective especially when an RDS receiver which has an AF search function is additionally provided with an FM multiplex demodulating function of a DARC system with a common front end.
Also according to the present invention, even when another station is searched while the receiver is not in synchronization with the data of the present station, the receiver is prevented from being synchronized at the data timing of another station. After completing the search, the receiver can be immediately synchronized with the present station. Therefore, the synchronized and received data immediately after the search is completed can be processed or errors can be corrected. Ineffective data can be minimized.